1. Field of Invention
This invention relates to a multi-speed power shift transmission, and more particularly, to a power shift transmission having two planetary sections for providing a plurality of forward and reverse speeds.
2. Description of Prior Art
Power shift transmissions have been used for many years to allow earthmoving and agricultural equipment to deliver the full power of an engine to the drive train without interruption by gear changes. Uninterrupted power flow is necessary to prevent a loss of speed during gear changes, which would occur rapidly under the heavy drag loadings of industrial and agricultural vehicles. Numerous arrangements of power shift transmissions have been set forth in the art to satisfy the varying needs of the vehicles into which they are incorporated. U.S. Pat. No. 4,420,992 and the references cited therein exemplify much of the art related to power shift transmissions. The major thrust of U.S. Pat. No. 4,420,992 and much of the prior art, has been to increase the number of speeds available in a power shift transmission.
The typical power shift transmission contains a series of planetary gear assemblies. These planetary gear assemblies comprise transmission subassemblies. In order to increase the available speeds in a power shift transmission, the prior art as shown the joining and interconnection of numerous transmission sub-assemblies. These planetary systems are coupled to perform speed changes through the selective connection or restraint of the planetary elements by a series of friction devices. Many of the power shift transmissions include complex planetaries or transmission subassemblies. One such planetary assembly is commonly referred to as a Ravigneaux planetary which typically consists of a ring gear, two sun gears and a pair of intermeshing planetary gear sets with one of the planetary gear sets meshing with one sun gear and the ring gear and the other planetary gear set meshing with the other sun gear. The connection of a Ravigneaux planetary to a simple planetary system is known from U.S. Pat. No. 4,345,490 and S.A.E. paper 8163, title "John Deere Power Shift Transmission" presented to the National Farm, Construction and Industrial Machinery meeting on Sept. 9-12, 1963. Although the interconnection of various planetary transmission subassemblies in power shift transmissions has effectively provided the various speeds required, the numerous and complex interconnections increase the size and expense of these transmissions while making assembly more difficult.
In addition to providing numerous speed ratios, the planetary assemblies and power shift transmissions have also been arranged to advantageously split power between the planetary elements. This power splitting could permit a reduction in the size of planetary elements due to a reduction in sustained loads on individual components. However, it is often the case that transmissions providing advantageous power splitting in certain gear ranges will recirculate power in other gear ranges. Recirculation of power multiplies loadings on transmission elements, thereby negating the advantage of power splitting in other gear ranges.
It is an objective of this invention to provide a power shift transmission that overcomes one or more of the aforementioned problems.
It is also an objective of this invention to provide a power shift transmission having a reduced number of planetary sections and interconnections between elements.
It is a further objective of this invention to provide a power shift transmission having six speeds, including reverse gears, using only two planetary sections and five friction elements.
A yet further objective of this invention is to provide a power shift transmission which provides advantageous power splitting in certain gear ranges and avoids power recirculation through transmission elements to allow a reduction in the size of the planetary assemblies.